Trees have many important functions within the urban environment including air quality improvement, wildlife habitat, and mitigation of the heat island effect; however, their contribution to green infrastructure, such as bioretention, is not well understood. There is a critical need to understand the health and role of trees in these systems to enable urban foresters and engineers to select the most appropriate plant material to maximize functionality of natural stormwater treatment systems. Though laboratory and field studies, the goal of this project was to demonstrate the role of trees in bioretention areas, and to make recommendations regarding system design and tree species selection to maximize both bioretention area functionality and tree health.